Field of the Invention
The present invention relates to a cathode material for a lithium-ion secondary battery and a lithium-ion secondary battery.
Description of Related Art
Lithium-ion secondary batteries have a higher energy density and a higher power density than lead batteries and nickel-hydrogen batteries and are used in a variety of applications such as small-size electronic devices such as smartphones, domestic backup power supply, and electric tools. In addition, attempts are made to put high-capacity lithium-ion secondary batteries into practical use for recyclable energy storage such as photovoltaic power generation and wind power generation.
Lithium-ion secondary batteries include a cathode, an anode, an electrolytic solution, and a separator. As electrode materials that constitute cathodes, lithium-containing metal oxides having properties capable of reversibly intercalating and deintercalating lithium ions such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), and lithium iron phosphate (LiFePO4) are used, and studies are made in order for improvement from the viewpoint of an increase in the capacity of batteries, the extension of service lives, improvement of safety, and cost reduction.
Lithium iron phosphate (LiFePO4) as the electrode material contains iron which is an abundant and inexpensive resource and is thus a material the cost of which can be easily reduced. Lithium iron phosphate does not emit oxygen at high temperatures due to the strong covalent bond between phosphorus and oxygen and thus has outstanding safety, which provides lithium iron phosphate with excellent characteristics that oxide-based cathode materials represented by lithium cobalt oxide do not have.
On the other hand, lithium iron phosphate has low Li ion diffusivity and low electron conductivity and thus has worse input and output characteristics than oxide-based cathode materials. This characteristic difference becomes more significant as the operation temperature of batteries becomes lower, and thus lithium iron phosphate has been considered to be inappropriate for in-vehicle applications such as hybrid vehicles for which high input and output characteristics are required at low-temperature regions.
LiMPO4 (M represents a metal element) having an olivine structure which is represented by lithium iron phosphate has low Li ion diffusivity and low electron conductivity, and thus it is possible to improve the charge and discharge characteristics by miniaturizing LiMPO4 primary particles and coating the surfaces of the respective primary particles with a conductive carbonaceous film.
On the other hand, since the miniaturized LiMPO4 has a large specific surface area, an increase in the viscosity of an electrode mixture slurry or a large amount of a binder is required, and thus it is usual to improve the properties of the electrode mixture slurry by turning the primary particles coated with a carbonaceous film into secondary particles by means of granulation.
For example, Japanese Laid-open Patent Publication No. 2012-243710 discloses a cathode for a non-aqueous electrolyte battery in which the surface of the cathode is flattened by dispersing cathode active material particles produced using a spray-dry method in a secondary particle form in a mixture of the cathode active material particles and solid electrolyte particles and crushing the exposed portions of the cathode active material particles that are exposed on the surface of the cathode by means of calendering.